This invention relates generally to an improved method for increasing product yield during the manufacture of crystalline magnesia particulates and more specifically to the benefits derived by modifying the cooling rate for a fused magnesia ingot being discharged from an electric furnace at extremely elevated temperatures.
Fused magnesia particulates are conventionally produced in an electric arc furnace forming a solid ingot from the molten starting material which is thereafter mechanically crushed and ground by various means to a final desired particle size. A wide assortment of fused magnesia products can be manufactured in such manner to include various types and grades for dissimilar end use applications. To further illustrate such diversity, this material is now commonly employed in electrical heating elements, refractory structures, and friction brake constructions. The electrical grade material for heating elements typically ranges in magnesia purity from at least 50 weight percent up to approximately 98 weight percent, with a grain size ranging from approximately 20 to 40 weight percent in the size range minus 40 mesh United States screen size, while further exhibiting required electrical resistance and thermal conductivity characteristics. A still further requirement for said electrical heating element application is mechanical flow of the fused magnesia particulates when filling the metal casings employed for said elements which can be enhanced by coating the magnesia particles with a solid or liquid lubricant, such as a silicone and the like. As distinct therefrom, a typical fused magnesia product for refractory structures such as bricks and blocks maintains a relatively high purity magnesia content in the 95-98 weight percent range together with a particle size from 80-2000 microns. The physical properties commonly specified for fused magnesia products to replace asbestos in friction brake constructions includes high temperature stability and Mohs hardness of at least 5. To satisfy these requirements a typical product ranges in magnesia content from approximately 90-99 weight percent with a particle size in the minus 40 plus 60 mesh United States screen size from approximately 25-35 weight percent.
A fused magnesia product suitable for electrical heating elements can be produced in the above manner utilizing a conventional submerged arc electric furnace of the general type long employed for steel-melting. Electric heating is supplied with dual or multiple electrodes to establish a reaction zone within the fusion vessel where the mineral charge becomes melted. In doing so, fusion can commence upon a batch charge already present in the furnace vessel with additional mineral batches being charged during the fusion process. When the charge material has been converted to a molten state, the applied electrical power is terminated allowing the furnace contents to cool from the extremely elevated furnace discharge temperatures providing a solid ingot with varying degrees of recrystallation. The furnace ingot is subsequently crushed by conventional mechanical means such as with a jaw crusher followed by a hammer mill which can still further include magnetic separation means to remove any entrained metal contaminants. A conventional vibrating screen apparatus is next employed to provide the desired particle size from the crushed material while further yielding varying amounts of undersized particles passing through 325 mesh screen size. The desired particle size material is then heated in an oxidizing atmosphere to produce approximately 1000.degree. C. for additional impurity removal and optionally blended with a suitable lubricant to provide a free-flowing powder having the above defined electrical and physical characteristics suited for heating element utilization.
The above described manufacturing process results in a solid ingot being discharged from the electric furnace at extremely elevated temperatures which is of a non-uniform composition. The innermost central region of the discharged ingot remains partially molten for recrystallization upon cooling to the recrystallization temperature with the outer peripheral regions surrounding the central core region consisting only of sintered or partially fused magnesia crystals. By slowing temperature loss in the discharged ingot upon its removal from the electric furnace it becomes possible not only to enhance additional recrystallization in the central core region of said ingot but to also enhance further melting in its outer peripheral regions. Such reduction in heat loss when cooling to ordinary ambient temperatures can thereby promote a desirable increase in the ingot size with additional recrystallized and sintered magnesia crystals being formed during the extended cooling period. A still further benefit promoted by slowing the ingot cooling rate from the extremely elevated furnace discharge temperatures is an improved chemical purity of the recrystallized magnesia crystals. Such recrystallized material will generally have fewer contaminants due to a refining process involving volatilization and migration of contaminants from the central core region of the cooling ingot. It remains desirable, therefore, to provide simple and effective means whereby final product yield of the resulting particulate mixture of recrystallized and sintered magnesia particulates can be achieved in the desired manner.
It is one object of the present invention, therefore, to modify the preparation of fused magnesia crystalline particulates so that increased product yield having a higher purity results.
It is another object of the present invention to provide novel process means for a more effective preparation of fused magnesia crystalline particulates from the starting material.
Still another object of the present invention is to provide novel means when fusing a bulk mass of magnesia mineral to promote larger amounts of both recrystallization and sintered crystal formation.
These and still further objects of the present invention will become apparent upon considering the following detailed description of the present invention.